The present invention relates to a biomedical implant material which constitutes an artificial bone or artificial joint used in orthopedic surgery of a patient having bone function or limb joint function lost due to a disease or a disaster, or an artificial tooth root used in a reconstructive surgery for a tooth lost due to aging or a disease, and a method of producing the same and, more particularly, to a biomedical implant material made of ceramics and a method of producing the same.
The field of implantology has been making remarkable advancements in recent years, and various artificial organs have been used in medicine such as artificial heart, artificial blood vessel and artificial lung. In the field of orthopedics, in particular, artificial joints are widely used for the restoration of joint functions that have been lost, while the artificial tooth root is attracting much attention in the field of dentistry.
Since intraosseous biomedical implants such as artificial joint, artificial bone and artificial tooth root must have high strength, such materials as stainless steel, cobalt-chromium alloy and titanium alloy have been mainly used, while ceramic materials have recently been used increasingly widely such as alumina and zirconia that have excellent corrosion resistance and sliding characteristic in the living body.
Such biomedical implants may be fixed in bone either by bonding to the bone by means of an acrylic resin called bone cement, or by making use of the restoring function of the patient""s bone tissue in a practice called the cementless implant surgery. The former is simple and easily practiced with favorable result of fixation and allows the patient to leave the bed early, but involves the possibility of necrosis of the bone tissue due to the toxicity of acryl monomer and the heat generated during polymerization. The latter is free from the problem of toxicity and is regarded as a preferable method of fixation that is more compatible to the organism, and also because the natural function of the organism is made use of, but makes the surgical operation more difficult while stable fixation cannot be easily achieved.
In order to improve the intraosseous fixation of cementless implant, use of metallic implants with porous coating or hydroxyapatite coating applied to the surface thereof at the interface with the bone has recently been increasing, with satisfactory clinical results.
As to the biomedical implants made of ceramics, on the other hand, techniques of coating with calcium phosphate materials have been developed to improve the intraosseous fixation of cementless implant. For example, Japanese Published Examined Patent Application (Kokoku Tokkyo Koho Sho No.) 59-46911 discloses a technique of coating an implant made of ceramics with a calcium phosphate material by plasma spraying, and Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei No.) 6-233782 discloses a technique of providing a titanium thermal-sprayed layer as an intermediate layer between a ceramic implant and a calcium phosphate thermal-sprayed layer, although practically satisfactory results have not been achieved.
The former technique has such drawbacks as the calcium phosphate layer formed by thermal spraying has an insufficient strength of bonding to the base material of ceramics and the mechanical strength of the ceramic substrate decreases. The latter technique has a great effect of improving the bonding strength of the calcium phosphate thermal-sprayed layer, but often fails to achieve a satisfactory mechanical strength to counter the decrease in the strength of the ceramic substrate.
Reasons of the above problems will be described below.
In the case of the former technique, thermal spraying processes such as plasma spraying require a preliminary process such as sand blast to roughen the surface, but the ceramic substrate is too hard to roughen the surface thereof. As a result, sufficient bonding strength between the calcium phosphate thermal-sprayed layer and the implant can not be achieved. A ceramic material is also brittle and the mechanical strength thereof may be decreased significantly by even a small flaw on the surface thereof. Also the sand blast process described above is an effective method of roughening the surface of metallic materials which are ductile, but may be harmful to the ceramic materials.
In the case of the latter technique, the decrease in the strength of the ceramic substrate is caused by microscopic cracks generated in the ceramic substrate due to the difference in the thermal expansion coefficient between ceramics and titanium, thermal impact during thermal spraying and/or quick solidification and shrinkage of the titanium thermal-sprayed layer. While grooves may be formed on the ceramic substrate with this technique in order to achieve higher bonding strength between the ceramic substrate and the titanium thermal-sprayed layer, sufficient force of fixation may not be achieved depending on the direction of forming the grooves.
In light of the problems of the prior arts described above, an object of the present invention is to provide a biomedical implant material made of ceramics having a coating layer formed thereon with a sufficient bonding strength without decreasing the mechanical strength of the ceramic substrate, and a method of producing the same.
The present invention has been completed in order to solve the problems described above.
The present invention relates to a biomedical implant material comprising a substrate for biomedical implant made of a ceramic material, a first coating layer formed on the surface of said substrate by low-thermal impact coating process, and a second coating layer formed on said first coating layer via a metallic layer formed by thermal spraying process, and
a method of producing a biomedical implant material, which comprises coating a substrate made of a ceramic material with titanium by the PVD (physical vapor deposition) process to form a titanium layer, thermal-spraying titanium on said titanium layer to form a titanium thermal-sprayed layer, and thermal-spraying a calcium phosphate layer on said titanium thermal-sprayed layer to form a calcium phosphate material layer.
The first coating layer of the biomedical implant of the present invention is formed by the low-thermal impact coating process, and is therefore provided without causing cracks in the substrate. The first coating layer has a function of shielding against the heat generated during thermal spraying of the metallic layer thereby protecting the substrate from significant thermal impact, and also serves as a bonding layer between the metallic layer and the ceramic substrate to hold both portions firmly together. This effect is achieved because, in contrast to a case of forming the metallic layer by the thermal spraying in which case the bonding strength between both members tends to be low as described above, the low-thermal impact coating process achieves high bonding strength and the bonding strength between the substrate and the metallic layer formed by the thermal spraying can be increased remarkably when the metallic layer is formed via the first coating layer by thermal spraying.
The biomedical implant material of the present invention also has the second coating layer formed via the metallic layer from the calcium phosphate material having the biocompatibility, with the surface thereof being roughened by thermal spray process (for example, plasma spraying or flame spraying), and therefore mechanical anchoring effect provides strong bonding force of the second coating layer to the first coating layer.
According to the present invention, since the intermediate layer (the first coating layer) is provided by the low-thermal impact coating process between the ceramic substrate and the thermal-sprayed layer made of titanium or the like (metallic layer), cracks are prevented from occurring in the ceramic substrate and sufficient bonding strength can be ensured between the thermal-sprayed layer made of titanium or the like and the ceramic substrate, thereby to provide a practically useful biomedical implant.